Fix major.minor version resolution in shims when using go.mod

[Copilot]

When GOENV_GOMOD_VERSION_ENABLE=1 is set and go.mod specifies a major.minor version (e.g., go 1.25), shim execution fails with "command not found" even when the corresponding patch version (e.g., 1.25.4) is installed.

# go.mod contains: go 1.25
# version 1.25.4 is installed

goenv exec go  # Fails: goenv: 'go' command not found

Root cause: goenv-version-name resolved "1.25" → "1.25.4" internally but never returned the resolved version, leaving shims with no version to execute.

Changes

libexec/goenv-version-name

• Add resolved version to output array when GOENV_GOMOD_VERSION_ENABLE is set
• Previously discarded the resolved version after successful version_exists check

libexec/goenv-which

• Pre-resolve major.minor versions before searching for executables
• Align version_exists implementation with goenv-version-name (handle both major.minor and major.minor.patch)
• Standardize on GOENV_GOMOD_VERSION_ENABLE variable (was inconsistently using GOENV_GO_MOD_ENABLE)

Tests

• Cover major.minor resolution from go.mod and direct GOENV_VERSION
• Cover shim execution path via goenv-exec
• Verify full versions (1.25.3) aren't re-resolved to latest patch

Original prompt

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This section details on the original issue you should resolve

<issue_title>Issue with major.minor shims not resolving correctly</issue_title>
<issue_description>After running

# GOENV_GO_MOD_ENABLE and GOENV_AUTO_INSTALL are both enabled here
goenv 
eval "$(goenv init -)"

Using latest patch version 1.25.4
Downloading go1.25.4.linux-amd64.tar.gz...
-> go1.25.4.linux-amd64.tar.gz
Installing Go Linux 64bit 1.25.4...
Installed Go Linux 64bit 1.25.4 to /root/.goenv/versions/1.25.4

# we experience a failure here
/root/.goenv/shims/go 
goenv: 'go' command not found 

The 'go' command exists in these Go versions:
  1.20.14
  1.21.13
  1.22.12
  1.23.12
  1.24.8
  1.25.4

We need to address this. Basically my go mod has go defined as go 1.25.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Issue with major.minor shims not resolving correctly #498

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